Bearing unit for wheel drive

ABSTRACT

An inner race  5  is retained by a crimped portion  30 , so that a pre-load is applied to rolling elements  6 . A bearing unit for wheel support  1  is connected to a constant velocity joint  2  through tightening of a nut  24 . By regulating the tightening force of the nut  24 , at the area of contact between the inside end of the crimped portion  30  and the outside end of the outer ring of the constant velocity joint  14 , the load per unit length is controlled up to 125 N/mm, or the surface pressure is controlled up to 1.5×10 8  Pa. Consequently, a bearing unit for wheel support is realized at a low cost with the pre-load securely applied, preventing unpleasant noise from occurring during operation, and preventing play due to plastic deformation.

TECHNICAL FIELD OF THE INVENTION

[0001] This invention relates to a bearing unit for wheel drive, andmore particularly to a bearing unit for wheel drive which integrates abearing unit for supporting the wheels with a constant-velocity joint,and which is used for supporting the drive wheels that are supported byan independent suspension {front wheels of a FF vehicle (front engineand front-wheel drive vehicle), the rear wheels of a FR vehicle (frontengine and rear-wheel drive vehicle), and all the wheels of a 4WDvehicle (four-wheel drive vehicle)} such that they rotate freely withrespect to the suspension, as well as for rotating and driving the drivewheels.

BACKGROUND TECHNOLOGY OF THE INVENTION

[0002] Various kinds of bearing units for wheel support, which areconstructed such that an outer race and inner race rotate freely by wayof rolling elements, have been used in order to support wheels such thatthey can rotate freely with respect to the suspension.

[0003] Moreover, a bearing unit for supporting the drive wheels on theindependent suspension, as well as for rotating and driving the drivewheels, is constructed by combining a bearing unit for supporting thewheels with a constant-velocity joint. This bearing unit for wheel drivemust be able to transmit the rotation of the drive shaft to the wheelssmoothly (securing constant velocity) regardless of the relativedisplacement between the differential gear and driven wheels and thesteering angle applied to the wheels.

[0004]FIG. 1 shows a typical bearing unit for wheel drive for thispurpose that comprises a bearing unit 1 for wheel support and aconstant-velocity joint 2.

[0005] In the bearing unit 1 for wheel support, a hub 4 and inner race 5are supported on the radial inside of an outer race 3 by way of aplurality of rolling elements 6 such that they rotate freely. Of these,the outer race 3 is fastened to the knuckle (not shown in the figure) ofthe suspension by a first flange 7, which is formed around the outerperipheral surface of the outer race 3, such that it does not rotateeven during use. In addition, a pair of outer-ring raceways 8 are formedaround the inner peripheral surface of the outer race 3, the hub 4 andinner race 5 are supported on the radial inside of this outer race 3such that they are concentric with the outer race 3 and such that theyrotate freely.

[0006] A second flange 9 for supporting the wheel is formed around theouter peripheral surface of the hub 4 at a portion near the outside endthereof (the outside is the side which is on the outside in the widthdirection of the vehicle when the bearing unit is installed in thevehicle, and in the figures that show the bearing unit for wheelsupport, including FIG. 1, it is on the left side). Also, a firstinner-ring raceway 10 is formed around the outer peripheral surface inthe middle of the hub 4, and a small-diameter stepped section 11 isformed around the outer peripheral surface on the inside end of the hub4 (the inside is the side which is on the center in the width directionof the vehicle when the bearing unit is installed in the vehicle, and inthe figures that show the bearing unit for wheel support, including FIG.1, it is on the right side), and the inner race 5 is fitted around thesmall-diameter stepped section 11 and fastened to this small-diameterstepped section 11 and a second inner-ring raceway 12 is formed aroundthe outer peripheral surface of the inner race 5. In addition, a firstspline hole 13 is formed in the center of the hub 4.

[0007] On the other hand, the constant-velocity joint 2 comprises anouter ring 14 for the constant-velocity joint, an inner ring 15 for theconstant-velocity joint, a plurality of balls 16 and a spline shaft 17.The outer ring 14 for the constant-velocity joint and the spline shaft17 form a drive-shaft member 18. In other words, this spline shaft 17 isformed on the outer half of the drive-shaft member 18, and it is fittedin the spline hole 13, while the outer ring 14 for the constant-velocityjoint is formed on the inner half of the drive-shaft member 18. Outsideengagement grooves 19 are formed at a plurality of locations in thecircumferential direction around the inner peripheral surface of theouter ring 14 for the constant-velocity joint, such that the outsideengagement grooves 19 are orthogonal to this circumferential direction.

[0008] Moreover, a second spline hole 20 is formed in the center of theinner ring 15 for the constant-velocity joint, and the inside engagementgrooves 21 are formed around the outer peripheral surface of the innerring 15 in alignment with the outside engagement grooves 19, andorthogonal with respect to the circumferential direction.

[0009] The aforementioned balls 16 are held between the insideengagement grooves 21 and the outside engagement grooves 19 by aretainer 22 such that they roll freely along the engagement grooves 21,19.

[0010] The shape of the components of the constant-velocity joint 2 arethe same as those for the well known Rzeppa-type or Barfield-typeconstant-velocity joint, and are not related to this invention so adetailed explanation of them is omitted.

[0011] For the constant-velocity joint 2 described above and this kindof rolling bearing unit 1 for wheel support, the spline shaft 17 isinserted into the spline hole 13 from the inside toward the outside.Also, on the outside end of the spline shaft 17, a nut 24 screws onto amale screw section 23 that is formed on the part that protrudes from theoutside end of the hub 4, and by tightening this nut 24, the splineshaft 17 and hub 4 are fastened together. In this state, the inside endsurface of the inner race 5 comes in contact with the outside endsurface of the outer ring 14 for the constant-velocity joint, so theinner race 5 does not move in a direction such that it would come apartfrom the small-diameter stepped section 11. At the same time, adequatepre-stressing is applied to the rolling elements 6.

[0012] Furthermore, when assembled with the suspension of the vehicle,the male spline section 26 that is formed on the outside end of thedrive shaft 25 is fitted in the second spline hole 20 that is formed inthe center of the inner ring 15 for the constant-velocity joint.

[0013] Also, the snap ring 28 that is fitted in the attachment groove 27that is formed all the way around the outer peripheral surface on theoutside end of the male spline 26 is engaged with the stepped section 29that is formed around the edge of the opening on the outside end of thesecond spline hole 20 to prevent the male spline section 26 from comingout from the second spline hole 20. The inside end of the drive shaft 25is fastened to the center of the trunnion of a tripod-typeconstant-velocity joint (not shown in the figure) that is formed on theoutput shaft of the differential gear (not shown in the figure).Accordingly, the drive shaft 25 rotates at constant velocity when thevehicle is running.

[0014] In the case of the bearing unit for wheel drive shown in FIG. 1,the force that presses the inner race 5 so as to apply a pre-load to therolling elements 6 that are located between the first and secondinner-ring raceways 10, 12 and each of the outer-ring raceways 8, isobtained by screwing the nut 24 onto the male screw section 23 andtightening it. Therefore, it is necessary to tighten the nut 24 verytightly to secure the pressure force of the inner race 5.

[0015] The size of the shaft force that occurs in the spline shaft 17and that presses the outside end surface of the outer ring 14 of theconstant-velocity joint against the inside end surface of the inner race5 by tightening the nut 24 on the male screw section 23 is substantiallylarge and depends on the size of the bearing unit for wheel drive, suchthat it is about (4 to 9)×10⁴ N for a normal passenger car.

[0016] Due to this large shaft force, surface pressure is applied to thearea of contact between the inside end surface of the inner race 5 andthe outside end surface of the outer ring 14 of the constant-velocityjoint, however, both of these end surfaces are a planar surface thatexists in a direction perpendicular to the center axis, and thereforecome in contact over a wide area with each other. Therefore, neither ofthe end surfaces have plastic deformation at the area of contact.

[0017] Moreover, with the construction disclosed in Japanese PatentPublication No. Tokukai Hei 11-5404, and as shown in FIG. 2, thecylindrical section existing on the inside end of the hub 4 at a portionthat protrudes inward beyond the inner race 5 that is fitted over thesmall-diameter stepped section 11 forms a crimped section 30 that isdeformed outward in the radial direction, and the inner race 5 isretained toward the stepped surface 31 of the small-diameter steppedsection 11 by this crimped section 30. In the case of this secondexample of prior construction, a pre-load is applied to the rollingelements 6 by the retaining force of the crimped section 30. Similar tothe first example of prior construction described above, the connectionbetween the bearing unit 1 for supporting the wheels and theconstant-velocity joint 2 is obtained by screwing the nut 24 onto themale screw section 23 that is formed on the outside end of the splineshaft 17 and tightening it. When this nut 24 is tightened, the flatsurface 32 that is formed on the inner surface of the crimped section 30comes in contact with the surface on the outside end of the outer ring14 of the constant-velocity joint. In the case of this second example ofprior construction as well, the aforementioned flat surface 32 is formedsuch that shaft force that occurs in the spline shaft 17 when the nut 24is screwed onto the male screw section 23 and tightened can become largeas in the case of the first example of prior construction.

[0018] With the bearing unit for wheel drive as shown in FIG. 1 and asdescribed above, an unpleasant rubbing noise of squeal sometimesoccurred when the vehicles was moving. The occurrence of this unpleasantnoise is known to be caused when the area of contact between the surfaceon the inside end of the inner race 5 and the surface on the outside endof the outer ring 14 of the constant-velocity joint rub due tofluctuations in torque that is transmitted between the constant-velocityjoint 2 and the bearing unit 1 for supporting the wheel. In other words,the torque changes frequently due to repeated acceleration anddeceleration. Moreover, the spline shaft 17 that is formed on the sideof the constant-velocity joint 2 elastically deforms in the twistingdirection as torque is transmitted between the constant-velocity joint 2and bearing unit 1 for wheel support, and the amount of deformationtends to change frequently as the torque fluctuates.

[0019] Also, as the force, which causes the spline shaft 17 to deform inthe twisting direction, or the force, which tries to return the twistedspline shaft 17 to the original position, becomes larger than frictionacting on the area of contact, minute slipping occurs at this area ofcontact. In this case, as the friction force acting on the area ofcontact becomes large, the rubbing energy between the surface on theinside end of the inner race 5 and the surface on the outside end of theouter ring 14 of the constant-velocity joint becomes large due to theslipping, and cause an unpleasant noise.

[0020] In order to prevent the occurrence of this kind of unpleasantnoise, a film for reducing the friction was formed using grease,molybdenum disulfide, fluorine resin, or the like, on the area ofcontact between the surface on the inside end of the inner race 5 andthe surface on the outside end of the outer ring 14 of theconstant-velocity joint. By making the area of contact slippery, it ispossible to keep the rubbing energy between the surface on the insideend of the inner race 5 and the surface on the outside end of the outerring 14 of the constant-velocity joint small even when minute slippingoccurs, and thus it is possible to make it difficult for the unpleasantnoise to occur.

[0021] It is known that this kind of method is somewhat effective.However, the film for reducing the friction is not always durableenough, and it is difficult to maintain sufficient effect over a longperiod of time. Particularly, with construction of not sealing thecontact area with a seal ring, the period of time that the unpleasantnoise can be effectively reduced is limited.

[0022] It is also thought that in order to reduce the friction, thetightening force on the nut 24 can be loosened to reduce the surfacepressure between the surface on the inside end of the inner race 5 andthe surface on the outside end of the outer ring 14 of theconstant-velocity joint. However, in the case of the first example ofprior art construction shown in FIG. 1, a pre-load is applied to therolling elements 6 due to the tightening force of the nut 24, soapplication of this method is difficult. In the case of the secondexample shown in FIG. 2 as well, increasing the tightening force of thenut 24 as in the case of the first example is considered, so it is notpossible to suppress the occurrence of the unpleasant noise.

[0023] Furthermore, in the case of the first example of prior artconstruction shown in FIG. 1, the tightening force of the nut 24 withrespect to the male screw section 23 is increased in order to apply apre-load, so a large axial force occurs in the spline shaft 17.Therefore, the torque required for tightening the nut 24 becomes large,and it is not possible to avoid decrease in work efficiency ofassembling the bearing unit for wheel drive.

[0024] Taking into consideration that the same axial force that occurredin the first example also occurs in the second example of prior artconstruction shown in FIG. 2, a large flat surface 32 is formed on thesurface of the inside end of the crimped section 30, and therefore, thesame problem occurs. Moreover, in the case of the second example shownin FIG. 2, the cost increases due to the work of tightening the nut 24with a large torque.

DISCLOSURE OF THE INVENTION

[0025] Taking the above problems into consideration, an objective ofthis invention is to provide a bearing unit for wheel drive that isconstructed so that the unpleasant rubbing noise of squeal can beeffectively prevented over a long period of time.

[0026] Moreover, taking the above problems into consideration, anotherobjective of this invention is to provide a bearing unit for wheel drivewhich can be constructed at low cost and still be capable of sufficientpre-loading of the rolling elements even when the torque for tighteningthe nut is reduced.

[0027] The bearing unit for wheel drive of this invention comprises: adrive shaft member on which an outer ring of a constant-velocity jointis formed on the inside half, and a spline shaft is formed on theoutside half; a hub with a spline hole formed in the center in which thespline shaft is fitted, and which is rotated and driven via theconstant-velocity joint during use due to the engagement between thespline hole and spline shaft; a flange that is formed around the outerperipheral surface on the outside end of the hub for supporting andfastening a wheel to the hub; a first inner-ring raceway that is formeddirectly around the outer peripheral surface in middle of the hub or byway of an inner race that is separate from the hub; a small-diameterstepped section that is formed at the inside end of the hub such thatthe dimension of its outer diameter is smaller than the section that isformed with the first inner-ring raceway; an inner race that has asecond inner-ring raceway formed around the outer peripheral surfacethereof and that is fitted around the small-diameter stepped section; anouter race that has a pair of outer-ring raceways formed around theinner peripheral surface thereof such that they face the aforementionedfirst and second inner-ring raceways, and which is not rotated evenduring use; and a plurality of rolling elements that are located betweeneach of the outer-ring raceways and the first and second inner-ringraceways.

[0028] A cylindrical section is located on the section of the inside endof the hub that protrudes further inward than the inner race that isfitted around the small-diameter stepped section, and a crimped section,which is crimped outward in the radial direction, is formed on thecylindrical section such that this crimped section retains the innerrace against the surface of the step of small-diameter stepped section,and the inner race is fastened to the hub with a pre-load applied to therolling elements.

[0029] A nut is screwed onto a male thread portion provided on the tipend of the aforementioned drive-shaft member, and the surface on theoutside end of the hub comes in contact with the surface of the insideend of the nut, and the drive shaft member and the hub are connected toeach other by tightening the nut with the surface on the inside end ofthe crimped section being in contact with the surface on the outside endof the outer ring of the constant-velocity joint.

[0030] Moreover, a feature of the present invention is that a flatsurface is formed on the surface of the inside end of the crimpedsection such that the average value of the surface pressure at the areaof contact between the surface on the inside end of the crimped portionand the surface on the outside end of the outer ring of theconstant-velocity joint is 1.5×10⁸ Pa or less.

[0031] Another feature of the present invention is that the contactbetween the surface on the inside end of the crimped section and thesurface on the outside end of the outer ring of the constant-velocityjoint is line contact, and the load per unit length F/L_(a) that isobtained by dividing the axial force F, which is applied to the splineshaft when tightening the nut, by the circumferential length L_(a)around the average diameter of the area of contact between the surfaceon the inside end of the crimped section and the surface on the outsideend of the constant-velocity joint, is 125 N/mm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a cross sectional view to show a first example of theconventional structure.

[0033]FIG. 2 is a cross sectional view to show one half of a secondexample of the conventional structure.

[0034]FIG. 3 is a cross sectional view to show a first example of theembodiment of the present invention.

[0035]FIG. 4 is a perspective view of a cotter pin.

[0036]FIG. 5 is a cross sectional view to show a second example of theembodiment of the present invention.

[0037]FIG. 6 is a cross sectional view to show a third example of theembodiment of the present invention.

[0038]FIG. 7 is a view of Portion VII in FIG. 8 to explain the operationof the present invention.

[0039]FIG. 8 is a cross sectional view of a fourth example of theembodiment of the present invention.

[0040]FIG. 9 is a cross sectional view of a fifth example of theembodiment of the present invention.

[0041]FIG. 10 is a cross sectional view of a sixth example of theembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT TO WORK THE INVENTION

[0042] Similar to the second example of prior art construction shown inFIG. 2, the bearing unit for wheel drive of a first embodiment of theinvention comprises a drive-shaft member, hub, flange, first inner-ringraceway, small-diameter stepped section, inner race, outer race androlling elements.

[0043] On the inside half of the drive-shaft member there is an outerring for the constant-velocity joint, and on the outside half there is aspline shaft.

[0044] Also, a spline hole is formed in the center of the hub in whichthe spline shaft is fitted, such that the hub is rotated and driven byway of the constant-velocity joint during use.

[0045] In addition, the flange is formed around the outer peripheralsurface on the outside end of the hub in order to support and fasten thewheel to the hub.

[0046] Moreover, the first inner-ring raceway is formed directly aroundthe outer peripheral surface in the middle of the hub, or by way ofanother inner race that is separate from the hub.

[0047] Also, the small-diameter stepped section is formed on the insideend of the hub such that the dimension of its outer diameter is lessthan that of the section formed with the first inner-ring raceway.

[0048] In addition, the inner race has a second inner-ring racewayformed around the outer peripheral surface thereof, and is fitted overthe small-diameter stepped section.

[0049] Moreover, the outer race has a pair of outer-ring raceways formedaround the inner peripheral surface thereof such that they face thefirst and second inner-ring raceways, and it does not rotate during use.

[0050] Also, the rolling elements are arranged such that a plurality ofrolling elements are located between each of the outer-ring raceways andthe first and second inner-ring raceways.

[0051] In addition, a cylindrical section exists on the section of theinside end of the hub that protrudes further inward than the inner racethat is fitted over the small-diameter stepped section, and a crimpedsection that is crimped outward in the radial direction is formed on thecylindrical section. This crimped section retains the inner race againstthe surface of the step of the small-diameter stepped section, and theinner race is fastened to the hub with a pre-load applied to the rollingelements.

[0052] Moreover, the surface of the outside end of the hub comes incontact with the surface of the inside end of the nut that screws onto amale thread portion provided on the tip end of the drive-shaft member,and the hub is fastened to the drive-shaft member by tightening the nutwith the surface of the inside end of the crimped section being incontact with the surface of the outside end of the outer ring of theconstant-velocity joint.

[0053] In the bearing unit for wheel support, a flat surface is formedon the surface of the inside end of the crimped section, and preferablythe average surface pressure at the area of contact between the surfaceon the inside end of the crimped section and the surface on the outsideend of the outer ring of the constant-velocity joint should be 1.5×18⁸Pa or less, and more preferably 1.0×10⁸ Pa or less. With the bearingunit for wheel drive of this invention, constructed as described above,the surface pressure at the area of contact between the surface on theinside end of the crimped section and the surface on the outside end ofthe outer ring of the constant-velocity joint is kept low, so it ispossible to keep the friction energy between the inside end surface andthe outside end surface small, and thus it is possible to prevent theunpleasant noise from occurring. Also, unlike in the case of forming afilm for reducing the friction, it is possible to prevent the noise fromoccurring for a long period of time.

[0054] The aforementioned cylindrical section is not a perfectcylindrical shape in order that the crimping process can be performedeasily, but it is preferable that at least one of the surfaces, theinner surface or outer surface, is tapered, such that the thickness inthe radial direction becomes smaller going toward the inside end.

[0055] Moreover, unlike the second example of prior art construction asshown in FIG. 2, in a second embodiment of the present invention, thereis no flat surface formed on the surface of the inside end of thecrimped section, and it is preferable that the cross-sectional shape ofthe crimped section be left as is with a convex circular arch shape,such that there is line contact between the surface on the inside end ofthe crimped section and the surface on the outside end of the outer ringof the constant-velocity joint.

[0056] Also, in the bearing unit for wheel support of this invention, itis preferable that the load per unit length F/L_(a) that is obtained bydividing the axial force F, which is applied to the spline shaft whenthe nut is tightened, by the circumferential length L_(a) around theaverage diameter of the contact area between the surface on the insideend of the crimped section and the surface on the outside end of theouter ring of the constant-velocity joint, be 125 N/mm or less. Thevalue of this load per unit length F/L_(a) is particularly effectivewhen the cross-sectional shape of the surface on the inside end of thecrimped section is a convex circular arc shape with a radius ofcurvature of 5 mm or more, and the surface on the outside end of theouter ring of the constant-velocity joint is a flat surface. With thebearing unit for wheel drive of this invention, constructed as describedabove, the load per unit length F/L_(a) at the area of contact betweenthe surface on the inside end of the crimped section and the surface onthe outside end of the outer ring of the constant-velocity joint is keptlow, so there is no plastic deformation of either of the end surfacesmentioned above, even when the surface on the inside end of the crimpedsection is not a flat surface. Moreover, the pre-loading of the rollingelements is performed by retaining the surface on the inside end of theinner race with the crimped section that is formed on the inside end ofthe hub, so even when the tightening force of the nut is reduced inorder to make the load per unit length F/L_(a) smaller, there is nodisplacement of the inner race with respect to the hub in the directionthat the inner race comes apart from the small-diameter stepped section,and thus pre-loading can be performed effectively.

[0057] Next, the reason why it is possible to prevent plasticdeformation of both end surfaces mentioned above by keeping the load perunit length F/L_(a) at 125 N/mm or less will be explained with referenceto FIG. 7.

[0058] The radius of curvature R₃₀ of the cross-sectional shape of thevirtually flat surface of the crimped section 30 that is formed on theinside end of the hub 4, which includes the center axis of the hub 4, soas to support the surface on the inside end of the inner race 5, is 5 mmor more. The reason for this is, that when the work for forming thecrimped section 30 on the inside end of the steel hub 4 is performedusing inexpensive cold forging, damage such as cracking occurs due tothe processing work when the radius of curvature R₃₀ is small. In otherwords, in order to form a crimped section 30 with no damage due to coldforging, the radius of curvature R₃₀ must be kept at 5 mm or greater.

[0059] The cross-sectional shape of the crimped section 30 is not just asimple circular arc, but is a complex curved surface of a plurality ofcurved surfaces having different radii of curvature, and the section forwhich the radius of curvature R₃₀ must be kept at 5 mm or more is thesection that protrudes inward the most and comes in contact with thesurface on the outside end of the outer ring 14 of the constant-velocityjoint.

[0060] On the other hand, of the crimped section 30, the minimum valueof surface hardness of the inside end that comes in contact with thesurface on the outside end of the outer ring 14 of the constant-velocityjoint, is about Hv 300. In other words, the inside end of the hub 4, onwhich the crimped section 30 is formed based on the plastic deformationdue to cold forging, cannot be made too hard in order that plasticdeformation occurs easily (by a comparatively small force such thatdamage due to cracking does not occur).

[0061] Accordingly, when taking into consideration differences inproduct quality due to workmanship during processing and work-hardeningdue to the crimping process based on cold forging, the minimum value ofthe hardness of the inside end of the crimped section 30 is about Hv300.

[0062] On the other hand, when the minimum value of the surface hardnessof the inside end is kept at Hv 300 or greater, it becomes easy fordamage to the crimped section 30 due to cracking to occur, and so thiscannot be applied. Also, quench hardening after the crimped section 30has been processed would be difficult to employ when considering theadverse effect that it has on the neighboring inner race 5 and increasedprocessing cost.

[0063] Therefore, the minimum value of the surface hardness of theinside end of the crimped section 30 is about Hv 300.

[0064] Furthermore, the allowable stress of the steel used for the hub4, or in other words, the difficulty of plastic deformation, isdetermined by the surface hardness. As the value of the surface hardnessincreases, the allowable stress increases, or in other words, plasticdeformation becomes more difficult. The allowable stress for a surfacehardness of Hv 300 is about 950 Mpa.

[0065] Also, when two cylinders come in contact with each other, themaximum surface pressure Pmax on the contact surface can be found fromthe well known Hertz equation, or equation (1) shown below.$\begin{matrix}\begin{matrix}\lbrack {{Equation}\quad 1} \rbrack \\{P_{\max} = \sqrt{\frac{E}{\pi( {1 - \frac{1}{m^{2}}} )} \cdot \frac{\Sigma \quad \rho}{2} \cdot \frac{F}{L_{a}}}}\end{matrix} & (1)\end{matrix}$

[0066] The meaning of the symbols in Equation (1) is as follows:

[0067] E: Modulus of Longitudinal elasticity

[0068] The hub 4 and outer ring 14 of the constant-velocity joint of thebearing unit for wheel drive are both made of steel, so the modulus oflongitudinal elasticity is 206,000 MPa.

[0069] m: Poisson's number

[0070] For steel the Poisson's number is 10/3.

[0071] Σρ: Sum of the curvature of the two cylinders (1/R₁, 1/R2)

(=1/R ₁+1/R ₂)

[0072] Of the symbols R₁ and R₂ in this equation, R₁ is equal to theradius of curvature R₃₀ of the cross-sectional shape of the crimpedsection 30, and this radius of curvature R₃₀ is 5 mm as described above.Also, R₂ is equal to the radius of curvature of the cross-sectionalshape of the surface on the outside end of the outer ring 14 of theconstant-velocity joint, and R₂=∞, and 1/R₂=0.

[0073] F: Load applied in the direction normal to the two cylinders [N]

[0074] The axial load applied to the spline shaft 17.

[0075] L_(a): Circumferential contact length of the two cylinders [m]

[0076] When the average diameter of the contact area is taken to be D,L_(a)=πD.

[0077] The values determined for the bearing unit for wheel drive areentered into Equation (1), and in order that the surface pressure on thecontact surface is up to the allowable stress, Equation (2) as shownbelow is obtained. $\begin{matrix}\begin{matrix}\lbrack {{Equation}\quad 2} \rbrack \\{\sqrt{\frac{206\text{,}000 \times 10^{6}}{\pi( {1 - \frac{1}{( \frac{10}{3} )^{2}}} )} \cdot \frac{0.2 \times 10^{3}}{2} \cdot \frac{F}{L_{a}}} \leqq {950 \times 10^{6}}}\end{matrix} & (2)\end{matrix}$

[0078] By solving Equation (2) above to find the load per unit lengthF/La,

F/L _(a)≦1.25×10⁵ [N/m]=125[N/mm]

[0079] As can be seen from this result, by regulating the tighteningforce of the nut that is screwed on the male screw section that isformed on the outside end of the spline shaft in order to keep the loadper unit length F/L_(a) at 125 N/mm or less, there is no plasticdeformation of the inside end of the crimped section 30 and the surfaceon the outside end of the outer ring 14 of the constant-velocity jointwhen the nut is tightened, and thus the nut does not become loose due toplastic deformation. The calculation above was performed when the radiusof curvature R₃₀ of the cross-sectional shape of the inside end of thecrimped section 30 was taken to be 5 mm. However, normally, the crimpedsection 30 is processed such that this radius of curvature R₃₀ is atleast 5 mm. As long as the load per unit length F/L_(a) is 125 [N/mm] orless, then the surface pressure on the contact area does not exceed theallowable stress of the inside end of the crimped section 30 or of thesurface on the outside end of the outer ring 14 of the constant-velocityjoint.

[0080] Moreover, there is a relational expression between the torque atwhich the nut is tightened and the axial force that occurs in the splineshaft 17 due to the tightening of the nut, so when assembling thebearing unit for wheel drive, the torque is controlled, however theaxial force is not directly controlled.

[0081] The preferred embodiments of the invention will be furtherexplained with reference to the drawings. Here, the same codes will beused for identical sections.

[0082]FIG. 3 shows a first example of a first. embodiment of theinvention. A feature of the present invention is that it is possible toprevent unpleasant rubbing noise of squeal from occurring duringoperation, in the constructions where the inner ring 5 that is fittedover the small-diameter stepped section 11 formed on the inside end ofthe hub 4 is fastened to the small-diameter stepped section 11 by acrimped section 30 that is formed on the inside end of the hub 4, whilea suitable pre-load is applied to the rolling elements 6. Theconstruction of the fastened section of the inner race 5 is the same asthat of the second example of prior art construction shown in FIG. 2,and the construction of all other sections is the same as that of thefirst example of prior art construction shown in FIG. 1, so anyredundant explanation will be omitted or simplified, and thisexplanation will center only on the features of this invention andsections that are different from the prior art constructions previouslydescribed.

[0083] The female spline, which is formed around the inner peripheralsurface of the spline hole 13 that is formed in the center of the hub 4,is a so-called parallel spline, and is formed such that each of thespline teeth is parallel with the center axis of the hub 4. In contrast,the male spline, which is formed around the outer peripheral surface ofthe spline shaft 17 on the outside half of the drive-shaft member 18, isa so-called twisted spline, and is formed such that the direction of thespline teeth is inclined a little with respect to the direction of thecenter axis of the drive-shaft member 18.

[0084] When the spline shaft 17 is pressed inside the spline hole 13such in order to combine the bearing unit 1 for wheel support with theconstant-velocity joint 2, the spline shaft 17 and spline hole 13 arefitted together with a spline fit with no play in the direction ofrotation.

[0085] Also, a cylindrical section 33 is formed on the surface of theoutside end of the nut 24, which screws onto the male screw section 23that is formed on the outside end of the spline shaft 17, and notches 34are formed at an even number of location around this cylindrical section33 (6 locations in the example shown in the figure) such that they areevenly spaced around in the circumferential direction.

[0086] On the other hand, a through-hole 35 is formed in the outside endof the male screw section 23 in the section that matches with thenotches 34 when the nut 24 is screwed and tightened such that it passesthrough the male screw section 23 in the radial direction.

[0087] Also, when the nut 24 is screwed onto the male screw section 23and tightened the required amount, a split cotter pin 36 is insertedthrough the through-hole 35 and the pair of notches 34 that are lined upwith the openings on both ends of the through-hole 35, as shown in FIG.4.

[0088] The tip end of this split cotter pin 36 (the front end in FIG. 3,and the right end in FIG. 4), is spread open through crimping as shownby the dashed line in FIG. 4, such that it cannot come out from thethrough-hole 35 and notches 34. With this construction, the nut 24 isfixed in place with the specified tightening. The tightening force(tightening torque) of this nut 24 is regulated by the relationship withthe surface area S of the flat surface 32 that is formed on the surfaceof the inside end of the crimped section 30 such that the average valueof the surface pressure at the area of contact between the flat surface32 and the surface on the outside end of the outer ring 14 of theconstant-velocity joint is up to 1.5×10⁸ Pa (≠15 kgf/mm²). In thisexample, the outer diameter D₃₂ of the flat surface 32 is 51 mm, theinner diameter d₃₂ is 47 mm, and the tightening force of the nut 24 iskept such that the axial force F that is applied to the spline shaft 17by tightening the nut 24 is 40 KN (≠4 tf) or less.

[0089] Under the conditions above, the average value of the surfacepressure P_(av) at the area of contact between the both end surfaces isfound as follows. $\begin{matrix}{P_{av} = {{F/S} = {( {40 \times 10^{3}} )/\{ {{\pi ( {0.051^{2} - 0.047^{2}} )}/4} \}}}} \\{\quad {\approx {1.3 \times {10^{8}\quad\lbrack{Pa}\rbrack}}}}\end{matrix}$

[0090] In this example, the surface pressure at the area of contactbetween the surfaces on both ends is kept at 1.5×10⁸ Pa or less, so itis possible to suppress the unpleasant rubbing noise of squeal thatoccurs at this area of contact during operation.

[0091] In other words, since the surface pressure at the area of contactis kept low, it is possible to keep the rubbing energy between both ofthese surfaces low, and to prevent this kind of noise from occurringbetween these end surfaces, or keep it low enough such that it is notunpleasant, even when there is rubbing between both of the end surfacesdue to twisting deformation of the spline shaft 17 during operation.

[0092] Next, testing that was performed in order to learn how thesurface pressure affects the generation of noise is explained.

[0093] In the tests, by adjusting the tightening force of the nut 24,the average value of the surface pressure at the area of contact betweenboth end surfaces was changed to be 1.0×10⁸ [Pa], 1.5×10⁸ [Pa], 2.0×10⁸[Pa] and 2.5×10⁸ [Pa], and noise was generated at the area of contactfor all four cases by repeatedly applying a torque in both directions tothe outer ring 14 of the constant-velocity joint. The generated noisewas listened to by ear and the level of the noise was determined. Theresults are shown in Table 1 below. In Table 1, the ‘◯’ mark indicatesthat the level of the noise was low and hardly noticeable, the ‘X’ markindicates that the level of the noise was high and very noticeable, andthe ‘Δ’ indicates that the level of the noise level was in between thelevels indicated by the ‘◯’ mark and ‘X’ mark. TABLE 1 Average SurfacePressure [Pa] at Area of Contact Noise Condition 2.5 × 10⁸ X 2.0 × 10⁸ X1.5 × 10⁸ Δ 1.0 × 10⁸ ◯

[0094] As can be clearly seen from Table 1, by keeping the averagesurface pressure at the area of contact between both end surfaces at1.5×10⁸ [Pa] or less, it is possible to reduce to a certain extent theunpleasant rubbing noise of squeal that occurs during operation, and byfurther keeping it at 1.0×10⁸ [Pa] or less, it is possible to keep thatnoise at a low level that is hardly noticeable.

[0095] In order to keep the average surface pressure low, it isnecessary to keep the tightening force of the nut 24 low, and, in thecase of this invention, the inner race 5 is retained by the crimpedsection 30, so that there is no decreasing or loosening of the pre-loadapplied to the rolling elements 6 even when the tightening force is keptlow.

[0096] Moreover, in order to suppress the generation of theaforementioned noise, it is possible to regulate the maximum value ofthe tightening force of the nut 24, however, from the aspect ofsuppressing the generation of the noise, it is not necessary to regulatethe minimum value of this tightening force. However, this minimum valueis regulated from the fact that relative displacement in the axialdirection between the drive-shaft member 18 and the hub 4 is controlledso that it does not occur regardless of the thrust load applied to thedrive-shaft member 18 during operation. In other words, duringoperation, a force in the direction that would cause relativedisplacement in the axial direction between the drive-shaft member 18and the hub 4 is applied by the axial force that is generated in thetripod-type constant-velocity joint on the differential gear side, andfurther by the centrifugal force that occurs during revolution. Sinceboth the drive-shaft member 18 and hub 4 are controlled such that theydo not shift regardless of this kind of force, the minimum value of thetightening force of the nut 24 is maintained at about 3.5 KN, includingthe safety factor.

[0097] Next, FIG. 5 shows a second example of the first embodiment ofthe present invention. This example differs from the first exampledescribed above in that there are no special notches 34 (see FIG. 3)formed on the cylindrical section 33 that is formed on the surface onthe outside end of the nut 24. Instead, in this example, a concavegroove 37 is formed partially around in the circumferential direction onthe outer half of the male screw section 23 that is formed on theoutside end of the spline shaft 17. In the case of this example, whenthe nut 24 has been tightened by a specified amount, the part of thecylindrical section 33 that matches with the concave groove 37 isplastically deformed inward (crimped) in the radial direction and madeto be fitted in this concave groove 37 in an interlocking manner inorder to prevent the nut 24 from becoming loose. Also, in this example,a pair of inner races 5, 5 a are fitted around the hub 4, to form afirst inner-ring raceway 10 and second inner-ring raceway 12.

[0098] In the case of this example as well, the tightening force of thenut 24 is regulated by the relationship with the surface area S of theflat surface 32 that is formed on the surface on the inside end of thecrimped section 30 such that the average value of the surface pressureof the contact area between the flat surface 32 and the surface on theoutside end of the outer ring 14 of the constant-velocity joint is1.5×10⁸ Pa (≠15 kgf/mm²) or less. In this example, the outer diameterD₃₂ of this flat surface 32 is 53 mm, and the inner diameter d₃₂ is 50mm, and the tightening force of the nut 24 is controlled such that theaxial force F that is applied to the spline shaft 17 when the nut 24 istightened is 20 KN (≠2 tf) or less.

[0099] Under the conditions above, the average value of the surfacepressure P_(av) at the area of contact between the surfaces of both endsis found as follows. $\begin{matrix}{P_{av} = {{F/S} = {( {20 \times 10^{3}} )/\{ {{\pi ( {0.053^{2} - 0.050^{2}} )}/4} \}}}} \\{\quad {\approx {0.8 \times {10^{8}\quad\lbrack{Pa}\rbrack}}}}\end{matrix}$

[0100] In this way, in this example as well, the surface pressure at thecontact area between both end surfaces is kept at 1.5×10⁸ Pa or less, soit is possible to suppress the unpleasant rubbing noise of squeal thatoccurs at this area of contact during operation. The other constructionand functions are the same as those of the first example describedabove.

[0101] Next, FIG. 6 shows a third example of the first embodiment of thepresent invention. This example differs from the first example describedabove in that there are no special notches 34 (see FIG. 3) formed on thecylindrical section 33 that is formed on the surface on the outside endof the nut 24. Instead, in this example, a concave section 38 is formedon the surface on the outside end of the spline shaft 17, and theoutside end of this spline shaft 17 is formed into a cylindrical shape.In the case of this example, when the nut 24 is tightened by a specifiedamount, part of the section in the circumferential direction around theoutside end of the spline shaft 17 is plastically deformed (crimped)outward in the radial direction together with the cylindrical section 33to be fitted with the cylindrical section and to prevent the nut 24 frombecoming loose.

[0102] In the case of this example as well, the tightening force of thenut 24 is regulated by the relationship with the surface area S of theflat surface 32 that is formed on the surface on the inside end of thecrimped section 30 such that the average value of the surface pressureof the contact area between the flat surface 32 and the surface on theoutside end of the outer ring 14 of the constant-velocity joint is1.5×10⁸ Pa (≠15 kgf/mm²) or less. In this example, the outer diameterD₃₂ of this flat surface 32 is 50 mm, and the inner diameter d₃₂ is 46mm, and the tightening force of the nut 24 is controlled such that theaxial force F that is applied to the spline shaft 17 when the nut 24 istightened is 30 KN (≠3 tf) or less.

[0103] Under the conditions above, the average value of the surfacepressure P_(av) at the area of contact between the surfaces of both endsis found as follows. $\begin{matrix}{P_{av} = {{F/S} = {( {30 \times 10^{3}} )/\{ {{\pi ( {0.050^{2} - 0.046^{2}} )}/4} \}}}} \\{\quad {\approx {1.0 \times {10^{8}\quad\lbrack{Pa}\rbrack}}}}\end{matrix}$

[0104] In this way, in this example as well, the surface pressure at thecontact area between both end surfaces is kept at 1.5×10⁸ Pa or less, soit is possible to suppress the unpleasant rubbing noise of squeal thatoccurs at this area of contact during operation. The other constructionand functions are the same as those of the second example describedabove.

[0105]FIGS. 7 and 8 show a fourth example of a second embodiment of thepresent invention, wherein FIG. 7 shows part of FIG. 8. The feature ofthe invention described below is that the inner race 5, which is fittedover the small-diameter stepped section 11 that is formed on inside endof the hub 4, is fastened to this small-diameter stepped section 11 by acrimped section 30 that is formed on the inside end of the hub 4, and asuitable pre-load is applied to the rolling elements 6, and thecross-sectional shape of the crimped section 30 is arc shaped with noplastic deformation of the contact area between the inside end of thecrimped section 30 and the surface on the outside end of the outer ring14 of the constant-velocity joint.

[0106] The construction of the fastening section of the inner race 5 isthe same as that of the second example of prior art construction shownin FIG. 2, and the construction of the other sections is the same asthat of the first example of prior art construction shown in FIG. 1, sothis explanation will center only on the features of this invention andthe sections that are different from the prior art constructiondescribed above.

[0107] The hub 4 is made of carbon steel, specifically annealed S53C. Asshown by the diagonal grid hatching shown in FIG. 8, of the outerperipheral surface in the center of the hub 4, the section that comes insliding contact with the seal lip 43 on the base section of a secondflange 9, the section of the first inner-ring raceway 10, and thesection that extends from the outer half of the small-diameter steppedsection 11 around which the inner race 5 is fitted, to the steppedsurface 31, are quenched by heat treatment such as induction-hardening.

[0108] Of these, in order to improve the yield strength against momentloads applied to the second flange 9, and to improve the resistance tofriction wear due to rubbing with the seal lip 43, the base of thesecond flange 9 is hardened by quenching. Also, the section of the firstinner-ring raceway 10 is hardened by quenching in order to secure itsrolling fatigue life.

[0109] In addition, the section that extends from the outer half of thesmall-diameter stepped section 11 to the step surface 31 isquench-hardened in order that there is no plastic deformation even whena load is applied to the inner race 5 that is fitted around the outsideof the small-diameter stepped section 11.

[0110] On the other hand, the section formed with the crimped section 30on the inside end of the hub 4 is not quench-hardened but left pliableas is. The hardness of the cylindrical section on this inside end beforethe crimped section is formed is about Hv 220 to 280. In contrast tothis, after the crimped section 30 has been formed by cold forging, thehardness of the surface of the crimped section 30 is about Hv 320 due towork hardening.

[0111] As described above, in the case of this invention, the flatsurface 32 as in the case of the second example of prior artconstruction shown in FIG. 2 is not formed on the inside end surface ofthe crimped section 30, but the cross-sectional shape of this surface onthe inside end has a convex circular arc shape. In this embodiment, theradius of curvature R₃₀ of the cross-sectional shape of the section ofthe inside end of the crimped section 30 that comes in contact with thesurface on the outside end of the outer ring 14 of the constant-velocityjoint described later, is 5.5 mm. Also, the radius of curvature of thecorner R portions formed on the inner peripheral edge on the inside endsurface of the inner race 5 is normally about 2 to 6 mm, however, inthis embodiment it is 4 mm.

[0112] Moreover, the drive-shaft member 18, which is connected to thehub 4 by the nut 24, is also made of carbon steel, and the areasindicated by the diagonal grid hatching in FIG. 8 are hardened byquenching. Specifically, the surface on the outside end of the outerring 14 of the constant-velocity joint, and the section that extendsfrom the base of the spline shaft 17 to a portion of the center near thetip end, are quench-hardened by heat treatment such as inductionhardening, to increase the strength of the base end of the spline shaft17.

[0113] For the reason previously described, it is difficult to increasethe surface hardness of the surface on the inside end of the crimpedsection 30, however, it is easy to harden the surface on the outside endof the outer ring 14 of the constant-velocity joint. By quenching thesurface on this outside end, it is possible to increase the strength ofthe aforementioned base section, as well as make plastic deformation ofthis outside end surface more difficult. On the other hand, as describedlater, it is possible to prevent plastic deformation of the surface onthe inside end of the crimped section 30 by regulating the tighteningforce of the nut 24.

[0114] Similar to the embodiment shown in FIG. 3, this embodimentcomprises a male screw section 23, cylindrical section 33, notches 34,and through hole 35. In regards to these, the previous description ofFIG. 3 is referenced.

[0115] It is preferable that a thin screw e.g. with a pitch of about 1mm be used where the amount that the axial force of the nut 24 isincreased per rotation angle is small, so that the tightening axialforce is kept within the allowable range set (between the upper andlower limits) at a position where the notches 34 match with thethrough-hole 35.

[0116] Without forming a through-hole 35 beforehand, it is possible totighten the nut 24 by a specified amount first, and then to form athrough-hole in the outside end of the spline shaft 17 in the section inalignment with a pair of suitable notches 34 that can be selected foreasy work, and finally a cotter pin 36 can then be inserted through thatthrough-hole.

[0117] Furthermore, without forming notches in the cylindrical section33 of the outside end of the nut 24, it is possible to tighten the nut24 by a specified amount first, and then to form a through-hole at asuitable location that can be selected for easy work that penetratesthrough both the cylindrical section 33 and the outside end of thespline 17 in alignment, and finally a cotter pin 36 can then be insertedthrough that through-hole.

[0118] In either of these cases, instead of the somewhat troublesomework of forming the through-hole, it is possible to more finely adjustthe amount of tightening of the nut 24 than in the case of inserting thecotter pin 36 through the pre-formed notches 34 and through-hole 35.

[0119] In either case, the tightening force (tightening torque) of thenut 24 is regulated such that the load per unit length F/L_(a) at thearea of contact between the surface on the inside end of the crimpedsection 30 and the surface on the outside end of the outer ring 14 ofthe constant-velocity joint is 125 N/mm or less. In this example, thediameter of the area of contact, or in other words, the diameter D₃₀ ofthe section where the surface on the inside end of the crimped section30 protrudes inward the most in the axial direction, is 50 mm, and thetightening force of the nut 24 is regulated such that the axial force Fthat is applied to the spline shaft 17 when the nut 24 is tightened is15,000 N or less.

[0120] Under these conditions, the load per unit length F/L_(a) is foundas shown below.

F/L _(a)=15,000/(50π)≠99.5[N/mm]

[0121] In the case of this example, the load per unit length F/L_(a) atthe area of contact between both end surfaces is kept at 125 N/mm orless, so it is possible to prevent plastic deformation of the surface onthe inside end of the crimped section 30 and the surface on the outsideend of the outer ring 14 of the constant-velocity joint. Particularly,in this example, the radius of curvature R₃₀ of the cross-sectionalshape of the crimped section 30 is 5.5 mm, which is greater than thevalue for the radius of curvature R₃₀ (5 mm) found to meet theconditions F/L_(a)≦125 N/mm, so that it is possible to adequatelyprevent the aforementioned plastic deformation.

[0122] In order to keep the load per unit length F/L_(a) low, it isnecessary to keep the tightening force of the nut 24 low, however, inthis invention, the inner race 5 is retained by the crimped section 30,so that there is no decreasing or loosening in the pre-load applied tothe rolling elements 6 even when the tightening force is kept low.

[0123] Moreover, in order to prevent plastic deformation of the surfaceon the inside end of the crimped section 30 and the surface on theoutside end of the outer ring 14 of the constant-velocity joint, it ispossible to regulate the maximum value of the tightening force of thenut 24, however, from the aspect of suppressing the aforementionedplastic deformation, it is not necessary to regulate the minimum valueof this tightening force. However, this minimum value is regulated fromthe fact that relative displacement in the axial direction between thedrive-shaft member 18 and the hub 4 is controlled so that it does notoccur regardless of the thrust load applied to the drive-shaft member 18during operation. In other words, during operation, a force in thedirection that would cause relative displacement in the axial directionbetween the drive-shaft member 18 and the hub 4 is applied by the axialforce that is generated in the tripod-type constant-velocity joint onthe differential gear side, and further by the centrifugal force thatoccurs during turning. Since both the drive-shaft member 18 and hub 4are controlled such that they do not shift regardless of this kind offorce, the minimum value of the tightening force of the nut 24 ismaintained at about 3,500 N, including the safety factor. Also, thelower limit is not regulated by the aforementioned load per unit lengthF/La, but is regulated by the force F in the axial direction.

[0124] Next, FIG. 9 shows a fifth example of the second embodiment ofthe invention. This example differs from the fourth example describedabove in that no special notches 34 (see FIG. 8) formed in thecylindrical section 33 that is formed on the outside end of the nut 24.Instead, in this example, a concave groove 37 is formed partially aroundin the circumferential direction on the outer half of the male screwsection 23 that is formed on the outside end of the spline shaft 17. Inthe case of this example, when the nut 24 has been tightened by aspecified amount, the part of the cylindrical section 33 that matcheswith the concave groove 37 is plastically deformed inward (crimped) inthe radial direction and made to be engaged with this concave groove 37in an interlocking manner in order to prevent the nut 24 from becomingloose.

[0125] Also, in this example, a pair of inner races 5, 5 a are fittedaround the hub 4, to form a first inner-ring raceway 10 and secondinner-ring raceway 12. In addition, in this example, it is not necessaryto maintain the hardness of the hub 4 at the level required for theinner-ring raceways. Therefore, in this example, the hub 4 is made ofcarbon steel S50C. Also, as shown by the diagonal grid hatching in FIG.9, of the outer peripheral surface of the hub 4, the section extendingfrom the base of the second flange 9 to the section over which the pairof inner races 5, 5 a are fitted are hardened by quenching.

[0126] In the case of this example as well, the tightening force of thenut 24 is regulated such that the force per unit length F/L_(a) at thearea of contact between the surface on the inside end of the crimpedsection 30 and the surface on the outside end of the outer ring 14 ofthe constant-velocity joint is 125 N/mm or less. In this example, thediameter of the area of contact, or in other words, the diameter D₃₀ ofthe portion of the inside end surface of the crimped section 30 whichprotrudes inward the most in the axial direction, is 47 mm, and thetightening force of the nut 24 is regulated such that the axial force Fthat is applied to the spline shaft 17 when the nut 24 is tightened is18,000 N or less.

[0127] Under these conditions, the load per unit length F/L_(a) is foundas shown below.

F/L _(a)=18,000/(47π)≠122.0[N/mm]

[0128] In this example, the load per unit length F/L_(a) of the area ofcontact between both end surfaces is kept at 125 N/mm or less, so thatit is possible to prevent plastic deformation of the surface on theinside end of the crimped section 30 and the surface on the outside endof the outer ring 14 of the constant-velocity joint.

[0129] Particularly, in this example, the radius of curvature R₃₀ of thecross-sectional shape of the crimped section 30 is 6.0 mm, which isgreater than the value for the radius of curvature R₃₀ (5 mm) found tomeet the conditions F/L_(a)≦125 N/mm, so that it is possible toadequately prevent the aforementioned plastic deformation. The otherconstruction and functions are the same as those of the fourth exampledescribed above.

[0130] Next, FIG. 10 shows the sixth example of the second embodiment ofthe invention. This example differs from the fourth example or fifthexample in that the first flange 7 (see FIGS. 8, 9) is not formed aroundthe outer peripheral surface of the outer race 3, but rather this outerperipheral surface is just a cylindrical surface. Also, an installationhole 40 is formed in the knuckle 39 of the suspension, and the outerrace 3 is fitted inside the installation hole 40 and prevented fromcoming out of this installation hole 40 by a pair of snap rings fittedaround the inner peripheral surface on both ends of the installationhole 40. Moreover, this example also differs from the fourth exampledescribed above in that the notches 34 (see FIG. 8) are not formed inthe cylindrical section 33 that is formed on the outside end surface ofthe nut 24. Instead, a concave section 42 is formed on the outside endsurface of the spline shaft 17 and the outside end of the spline shaft17 is formed into a cylindrical shape.

[0131] In the case of this example, when the nut 24 is tightened by aspecified amount, part in the circumferential direction of the outsideend of the spline shaft 17 plastically deforms (is crimped) outward inthe radial direction together with the cylindrical section 33 to befitted with the cylindrical section 33 in an interlocking manner and toprevent the nut 24 from becoming loose.

[0132] In the case of this example as well, the tightening force of thenut 24 is regulated such that the force per unit length F/L_(a) at thearea of contact between the surface on the inside end of the crimpedsection 30 and the surface on the outside end of the outer ring 14 ofthe constant-velocity joint is 125 N/mm or less. In this example, thediameter of the area of contact, or in other words, the diameter D₃₀ ofthe portion of the inside end surface of the crimped section 30 whichprotrudes inward the most in the axial direction, is 45 mm, and thetightening force of the nut 24 is regulated such that the axial force Fthat is applied to the spline shaft 17 when the nut 24 is tightened is16,000 N or less.

[0133] Under these conditions, the load per unit length F/L_(a) is foundas shown below.

F/L _(a)=16,000/(45π)≠113.2[N/mm]

[0134] In this example, the load per unit length F/L_(a) of the area ofcontact between both end surfaces is kept at 125 N/mm or less, so thatit is possible to prevent plastic deformation of the surface on theinside end of the crimped section 30 and the surface on the outside endof the outer ring 14 of the constant-velocity joint. Particularly, inthis example, the radius of curvature R₃₀ of the cross-sectional shapeof the crimped section 30 is 6.5 mm, which is greater than the value forthe radius of curvature R₃₀ (5 mm) found to meet the conditionsF/L_(a)≦125 N/mm, so that it is possible to adequately prevent theaforementioned plastic deformation. The other construction and functionsare the same as those of the fourth and fifth examples described above.

[0135] In all of the examples of the preferred embodiments describedabove, ball were used for the rolling elements 6, however, in the caseof a bearing unit for wheel drive of a heavy vehicle, tapered rollersmay be used for the rolling elements. Of course, construction that usestapered rollers for the rolling elements is within the technical scopeof this invention.

[0136] Also, in the case of construction where a flat surface 32 isformed on the surface on the inside end of the crimped section 30 asshown in FIG. 2, it is possible to make the load per unit length F/L_(a)at the area of contact between the surface on the inside end of thecrimped section 30 and the surface on the outside end of the outer ring14 of the constant-velocity joint far greater than 125 N/mm. In thiscase, making the average value of the surface pressure at the area ofcontact between the surface on the inside end of the crimped section 30and the surface on the outside end of the outer ring 14 of theconstant-velocity joint 1.5×10⁸ Pa or less, is as described previouslyfor the first embodiment. In this case the maximum value for the loadper unit length F/L_(a) is found under the conditions when the surfacepressure at the area of contact is 950 Mpa as described previously withrespect to Equation 2.

[0137] Applicability to the Industry

[0138] This invention, which is constructed and functions as describedabove, provides a bearing unit for wheel drive that makes it possible toadequately apply a pre-load to the rolling elements, as well as suppressover a long period of time unpleasant noise that is generated duringoperation. Moreover, this invention provides a low-cost bearing unit forwheel drive that makes it possible to adequately apply a pre-load to therolling elements, as well as prevents play that occurs due to plasticdeformation.

1. A bearing unit for wheel drive comprising: a drive shaft memberhaving an inside half on which an outer ring of a constant-velocityjoint is formed, and an outside half on which a spline shaft is formed;a hub having a radially center portion formed with a spline hole inwhich the spline shaft is fitted, so as to be rotated and driven via theconstant-velocity joint during use; the hub having at an out side endthereof a peripheral surface formed with a flange for supporting andfastening a wheel to the hub; and at an inside end a small-diameterstepped section such that the dimension of the outer diameter of thestepped section is smaller than the section that is formed with thefirst inner-ring raceway; an inner race having an outer peripheralsurface formed with a second inner-ring raceway and fitted around thesmall-diameter stepped section; the hub having at an intermediateportion a peripheral surface provided with a first inner-ring racewaythat is formed directly around the outer peripheral surface or by way ofanother inner race that is separate from the hub; an outer race havingan inner peripheral surface formed with a pair of outer-ring raceways toface the first and second inner-ring raceways, and being not rotatedeven during use; and a plurality of rolling elements that are locatedbetween each of the outer-ring raceways and the first and secondinner-ring raceways, the inside end of the hub formed with a cylindricalsection that protrudes further inward than the inner race that is fittedaround the small-diameter stepped section, the cylindrical section beingcrimped outward in the radial direction to form a crimped section, suchthat this crimped section retains the inner race against the stepsurface of small-diameter stepped section, and the inner race beingfastened to the hub with a pre-load applied to the rolling elements, anut being screwed onto the tip end of the drive-shaft member andprovided with an inside end, the hub being connected and fixed to thedrive shaft member by tightening the nut with the surface on the outsideend of the hub in contact with the surface of the inside end of the nut,and with the surface on the inside end of the crimped section in contactwith the surface on the outside end of the outer ring of theconstant-velocity joint, wherein the average value of the surfacepressure at the area of contact between the surface on the inside end ofthe crimped portion and the surface on the outside end of the outer ringof the constant-velocity joint is up to 1.5×10⁸ Pa.
 2. A bearing unitfor wheel drive comprising: a drive shaft member having an inside halfon which an outer ring of a constant-velocity joint is formed, and anoutside half on which a spline shaft is formed; a hub having a radiallycenter portion formed with a spline hole in which the spline shaft isfitted, so as to be rotated and driven via the constant-velocity jointduring use; the hub having at an out side end thereof a peripheralsurface formed with a flange for supporting and fastening a wheel to thehub; and at an inside end a small-diameter stepped section such that thedimension of the outer diameter of the stepped section is smaller thanthe section that is formed with the first inner-ring raceway; an innerrace having an outer peripheral surface formed with a second inner-ringraceway and fitted around the small-diameter stepped section; the hubhaving at an intermediate portion a peripheral surface provided with afirst inner-ring raceway that is formed directly around the outerperipheral surface portion or by way of another inner race that isseparate from the hub; an outer race having an inner peripheral surfaceformed with a pair of outer-ring raceways to face the first and secondinner-ring raceways, and being not rotated even during use; and aplurality of rolling elements that are located between each of theouter-ring raceways and the first and second inner-ring raceways, theinside end of the hub formed with a cylindrical section that protrudesfurther inward than the inner race that is fitted around thesmall-diameter stepped section, the cylindrical section being crimpedoutward in the radial direction to form a crimped section, such thatthis crimped section retains the inner race against the surface of thestep of small-diameter stepped section, and the inner race beingfastened to the hub with a pre-load applied to the rolling elements, thedrive-shaft member having a tip end formed with a male thread portion, anut being screwed onto the male thread portion and provided with aninside end, the hub being connected and fixed to the drive shaft memberby tightening the nut with the surface on the outside end of the hub incontact with the surface of the inside end of the nut, and with thesurface on the inside end of the crimped section in line contact withthe surface on the outside end of the outer ring of theconstant-velocity joint, wherein the load per unit length F/L_(a) thatis obtained by dividing the axial force F, which is applied to thespline shaft when tightening the nut, by the circumferential lengthL_(a) around the average diameter of the area of contact between thesurface on the inside end of the crimped section and the surface on theoutside end of the constant-velocity joint, is up to 125 N/mm.